Re: [PATCH v1 1/2] vdso: Add VDSO_UNALIGNED_PACKED config option for ARMv5

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Oct 20, 2026, at 10:41, Marc Kleine-Budde wrote:
> On 20.10.2026 10:24:51, Arnd Bergmann wrote:
>> On Tue, Oct 20, 2026, at 08:44, Marc Kleine-Budde wrote:
>>
>> That would be better, but I still hope we can just get back to the
>> portable code across all architectures for the in-kernel version.
>>
>> I have one more request for you: can you check /proc/cpu/alignment
>> on the system that shows the performance regression with and without
>> the patch?
>
> I asked Stefan Kerkmann (who wrote the original regression report) the
> same question, but we haven't had time for it yet. We'll get around to
> it.

Ok, thanks!

>> In your original report, you have shown that Ian's patch
>> caused the drop in performance and that it caused the added function
>> calls, but I don't see any sufficiently hot code path that would
>> actually explain the scale of the regression,
>
> Don't underestimate the overhead of function calls on those old CPUs
> with tiny I-caches.

What I meant is that I tried to look for calls to get_unaligned()
in the code path you'd hit with UBIFS on nand flash, and I only
found a possible call once per page written
(ebu_nand_write_page_hwecc), which should have a much lower
overhead than what you observe.

While ubifs does use crc32, it uses the lib/crc/ version rather
than the crypto/crc32c.c that you listed, and there is no
get_unaligned() in the lib/ one. If you have NOR flash, the
caller is likely do_write_buffer() calling map_word_load().
That one could explain the problem and could also be
optimized further if it turns out to be the bottleneck.

     Arnd